1. Field of the Invention
This invention relates to power switches for interrupting current in electric power circuits. More particularity, it relates to molded cases for such switches having a removable secondary cover which encloses recesses for auxiliary devices in a primary cover, and quick release devices for removably securing the secondary cover over the primary cover.
2. Background Information
Power switches, such as circuit breakers, transfer switches and the like, used in low voltage (600 volts and below) electric power distribution systems commonly have a housing molded of an electrically insulative resin and are, therefore, referred to as molded case circuit breakers, transfer switches and the like. The molded housing includes a base and a cover which together form an enclosure in which are mounted one or more switch poles, a switch operating mechanism and, where appropriate, a trip device. Typically, the molded case circuit breaker has a trip device that responds to short circuits and usually to persistent over current conditions also, and opens the main switch contacts to interrupt the current.
Often, it is desirable to provide additional features on the switch, sometimes as options. Such additional features can include a remote trip, an auxiliary switch which provides an indication of the open or closed state of the main switch contacts, and/or a bell alarm which provides an indication that the trip device has been activated. In some installations, it is desirable to have the switch opened on under voltage or off frequency conditions to prevent damage to a protected load.
These additional features can be incorporated within the switch housing formed by the base and cover. However, in some instances, the devices providing the desired features are received in cavities formed in the cover of the housing. In this later case, a secondary cover encloses the cavities in what has become the primary cover. This secondary cover should be easily removable for inserting, removing or servicing the devices providing the additional features. Typically, the secondary cover is removably secured to the molded housing by screws. However, screws either require threaded inserts in the housing, which increases the cost and manufacturing steps required, or when self tapping screws are used, the threads in the molded resin can be stripped out with repeated insertion and removal. In either case, the screws can be lost when removed. One solution to the lost screw problem is to have fasteners that are retained in the secondary cover; however, these too add to the cost and manufacturing effort required.
There is a need, therefore, for an improved molded case power switch housing and particularly for an improved arrangement for removably attaching a secondary cover to the molded housing.
There is a further need for such an improved molded case power switch housing in which there are no loose fasteners to become lost or hard to locate.
There is an additional need for such an improved molded case power switch which eliminates the manufacturing steps and costs associated with providing tapped inserts or specialized fasteners captured on the secondary cover.
There is another need for such an improved molded case power switch that does not require repeated insertion and removal of self-tapping screws into the molded housing.
There is yet another need for an improved molded case power switch housing which is economical to manufacture and easy to use.